How to add new tag

I am trying to use a new tag in a docbook document. The tag is in a 
different namespace. However, it's not being matched in my customization 
layer.

In the document:

<directory-structure xmlns="http://directories">

In my stylesheet:

<?xml version="1.0"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    xmlns:fo="http://www.w3.org/1999/XSL/Format"
    xmlns:d="http://directories" version="1.0">
    <xsl:import href="/opt/docbook/fo/docbook.xsl" />
    <xsl:strip-space elements="*" />

    <xsl:template match="d:directory-structure">got it</xsl:template>


I am not able to get the tag "d:directory-structure".  Is there 
something else I am missing?